The present invention relates to an optical scanner and a method of controlling the optical scanner, and more particularly to an optical scanner which scans light beams by oscillating a movable member at intrinsic resonance frequency thereof and a control method of the optical scanner.
An optical scanner which is represented by a laser beam printer or a projector which projects an image by scanning light beams has been conventionally used in various industrial fields. Such an optical scanner adopts an optical scanning mechanism which changes the direction of light and, recently, as a type of the mechanism, an oscillation-type optical scanning mechanism has been attracting attentions from a viewpoint of miniaturization of the device.
In one conventional example of the oscillation-type optical scanning mechanism, light beams are scanned such that an oscillating body having a reflecting mirror portion is oscillated so as to change the reflection direction of light beams incident on the reflecting mirror portion.
The oscillating body is configured to include the reflecting mirror portion, a fixed frame portion, a resilient deformation portion made of a resilient material or the like which is connected to the reflecting mirror portion and the fixed frame portion, and a drive source which imparts the deformation to the resilient deformation portion. Here, as the drive source, a drive source which makes use of an electrostatic force, an electromagnetic force, an action of heat, a piezoelectric action or the like can be named.
Further, in many conventional optical scanners, all operations of the optical scanner including the optical scanning mechanism is managed and controlled based on a master clock which constitutes a basis of time-series processing. However, a frequency of this master clock is fixed. That is, image data is read from an external device based on the master clock, and light beams are scanned based on the master clock and hence, it is possible to acquire a proper amount of image data from the outside and, at the same time, the light beams can be scanned as a visible light spot at a spatial position where an image is expected by changing the radiation direction of the light beams.
Patent document 1 discloses in detail the optical scanner which uses the oscillation-type optical scanning mechanism for scanning light beams in the horizontal direction. The summary of the invention disclosed in patent document 1 is explained hereinafter in conjunction with FIG. 11.
An optical scanner 100 shown in FIG. 11 is configured to directly project an image on a retina of a viewer. The optical scanner 1 includes a light source unit part 101, a vertical scanning system 102, a horizontal scanning system 103 and relay optical systems 126, 127.
In the scanning optical system 103, an oscillation-type optical scanning mechanism 104 which is constituted by combining a piezoelectric element and an oscillating reflecting mirror is driven in response to a drive signal from a horizontal scanning drive circuit 121. When light beams are radiated to the reflecting mirror, scanned light beams are received by a beam detector (BD) 123 which detects a RD signal. Then, in a signal processing circuit 105 provided to a light source unit part 101, a horizontal synchronizing signal based on a master clock and the BD signal is generated. Further, in the horizontal scanning drive circuit 121, the above-mentioned drive signal is obtained using the horizontal synchronizing signal as the reference.
Here, it is considered desirable to set frequency for driving the oscillation-type optical scanning mechanism 104 (scanning frequency in the horizontal direction) to a value substantially equal to resonance frequency of a-movable member of the oscillation-type optical scanning mechanism 104 for stabilizing an oscillating state. Further, in a field of control technique, a technique which oscillates a movable member of an actuator at a resonance frequency for acquiring a large variable range in a miniaturized actuator is known (for example, see patent document 1). Patent document 1: JP-A-2004-191953